1. Field of the Invention
The invention relates generally to the field of implantable cardiac devices, and more particularly, to an implantable cardiac device (ICD) configured to analyze metabolic gases for the assessment of cardiac output.
2. Related Art
Many chronic diseases, such as diabetes and heart failure, require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up is unsatisfactory for some diseases, such as heart failure, in which acute, life-threatening exacerbations can develop between physician follow-up examinations.
Congestive heart failure (CHF) is a chronic disease characterized by frequent exacerbations leading to expensive hospitalizations. Indeed, a patient hospitalized with CHF has a 50 percent chance of being readmitted for the same reason within 6 months. It is well known that close, routine monitoring of these patients allows early, simple, and inexpensive medical intervention which can prevent the exacerbation and eliminate the need for hospitalization. Monitoring for signs of an impending exacerbation thus both improves clinical outcomes and significantly reduces the cost of caring for these patients. It is well known among clinicians that if a developing exacerbation is recognized early, it can be easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute heart failure exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of heart-failure that many patients succumb to the disease.
It is often difficult for patients to subjectively recognize a developing exacerbation, despite the presence of numerous physical signs that would allow a physician to readily detect it. This problem is well illustrated by G. Guyatt in his article entitled “A 75-Year-Old Man with Congestive Heart Failure,” 1999, JAMA, 281(24): 2321-2328. Furthermore, since exacerbations typically develop over hours to days, even frequently scheduled routine follow-up with a physician cannot effectively detect most developing exacerbations. It is therefore desirable to have a system that allows the routine, frequent monitoring of patients so that an exacerbation can be recognized early in its course. With the patient and/or physician thus notified by the monitoring system of the need for medical intervention, a developing exacerbation can easily and inexpensively be terminated early in its course.
The multiplicity of feedback mechanisms that influence cardiac performance places the heart at the center of a complex control network. The neurohumoral axis includes the autonomic nervous system, consisting of sympathetic and parasympathetic branches, and numerous circulating hormones such as catacholamines, angiotensin, and aldosterone. Neural reflex arcs originating from pressure and stretch receptors, which directly measure mechanical hemodynamic status, modulate the neurohumoral axis. Similarly, chemoreceptors respond to changes in CO2, pH, and O2, which reflect cardiopulmonary function. The neurohumoral system influences cardiac performance at the level of the cardiac electrical system by regulating heart rate and the conduction velocity of electrical depolarizations. It also influences cardiac performance at the mechanical level, by controlling contractility, that is, the effective vigor with which the heart muscle contracts. Conventional cardiac monitors, such as defibrillators, pacemakers, Holter monitors, and cardiac event records, are tailored for the diagnosis and/or therapy of abnormalities of the cardiac electrical system. In contrast, heart failure is a disease of the cardiac mechanical system. It is primarily a failure of the myocardium to meet the mechanical pumping demands required of it. In monitoring the status of a heart failure patient, measuring the mechanical hemodynamic variables is desirable. Examples of mechanical hemodynamic variables include atrial, ventricular, and arterial pressures, and cardiac output (volume of blood pumped into the aorta per unit time).
One approach to frequent monitoring of heart failure patients that has been proposed is the daily acquisition of the patient's weight and responses to questions about subjective condition (see, for example, Alere DayLink Monitor, Alere Medical, Inc., San Francisco, Calif.). The simplicity and noninvasive aspect of this approach are desirable features. However, both the amount and the sophistication of the objective physiological data that can be acquired in this way are quite limited, which consequently limits the accuracy of the system. Furthermore, the system requires the active participation of the patient, who must not deviate from the precise data acquisition routine or risk introducing confounding factors into the acquired data.
In another approach to monitoring cardiac patients, oxygen saturation or partial pressure sensors are placed in the right ventricle for rate responsive pacing, in which the pacing rate of the pacemaker is controlled based on the metabolic demand of the body, which is a form of hemodynamic assessment and pace-parameter optimization. Assuming arterial O2 is constant, a fall in venous O2 below a critical level implies that the cardiac output is not sufficient to meet metabolic demand. In this case, a pacing parameter, the pacing rate, is increased.
A number of examples of a variety of measures of hemodynamic status, including both implantable embodiments (cardiac output measured using impedance plethysmography of the right ventricular volume, and right ventricular pressure) and external embodiments (cardiac output measured using Doppler ultrasound, heart sounds, blood pressure, respiratory gas analysis, and pulse oximetry) are known. External measurements of hemodynamic status are labor-intensive and can only be used during periodic follow-up examination. They are therefore not suitable for arrhythmia discrimination, dynamic pace-parameter optimization, sensitivity optimization, or capture verification.
Non-invasive techniques, such as plethysmography of vasculature, are also known. These techniques provide the basis of the conventional pulse oximeter, which by using two wavelengths of light, calculates the percent of arterial hemoglobin that is saturated with oxygen. The light is typically directed through the fingertip using a temporarily applied finger sensor. It can also be directed through other fleshy appendages such as the ear and, in infants, the foot. Optical vascular plethysmography also provides the basis for a non-invasive, continuous blood pressure monitor. A cuff containing an optical source and detector is placed over the finger. The pressure in the cuff is continuously varied so that the amount of light measured at the detector remains constant, which indicates that the volume of the vasculature is constant. In this way the arterial pressure can be inferred from the cuff pressure that is necessary to maintain constant light detection. Thus, while optical plethysmography of the vasculature is known in the art, it has to date been configured mostly for temporary, external use.
What is needed is a technique for continuously measuring cardiac output safely and accurately, with minimum disruption to a patient's normal activities.